The present invention relates to a method for controlling an engine exhaust gas system using an oxygen sensor, a three-way catalyst and a nitrogen oxide sensor and more specifically to a method for controlling an engine exhaust gas system, which can accurately control the exhaust amount of nitrogen oxide (hereinafter, referred to as NOx) and further detect the deterioration of a three-way catalyst easily.
There has conventionally been employed a method for controlling an air fuel ratio and removing harmful matters such as NOx, carbon monoxide and hydrocarbon in exhaust gas of the internal engine of an automobile by using an oxygen sensor and a three-way catalyst. On account of dispersion in characteristics of the oxygen sensor and change with time in characteristics of the sensor itself, however, the control point varied, so that the size of a three-way catalyst and the amount of noble metal carried thereon needed securing to a little excess to restrict the discharge of harmful matters within a determined value. Alternatively, there has been a method for correcting dispersion in control point by placing an oxygen sensor also downstream of a catalyst.
Even according to any of these methods, however, there were drawbacks in that the air fuel ratio deviates from a setup value, a great amount of NOx, carbon monoxide (CO), hydrocarbon (HC) or the like is exhausted suddenly and the regulating value for exhaust gas is apt to be exceeded under transient operating conditions such as acceleration or deceleration. As for methods for detecting the deterioration of a catalyst, there have been proposed a method for detecting the deterioration as a change in the oxygen adsorption capacity of a catalyst in accordance with the amplitude or response time of output of an oxygen sensor by using the oxygen sensor disposed downstream of the catalyst, and a method for detecting the amount of unreacted components in accordance with an increase in temperature accompanying combustion by using a temperature sensor with the catalyst. However, these methods served to only indirectly detect the deterioration of a catalyst and accordingly had a low accuracy.
Furthermore, there is known a nitrogen oxide sensor described in European Patent Publication 0678740A1. This sensor was slower in response speed than a conventional concentration cell oxygen sensor and it was difficult to directly control the air fuel ratio of a three-way catalyst system by only this sensor.
Besides, Japanese Patent Publication No. 4-65224 discloses a system for controlling an air fuel ratio by using an oxygen sensor installed in front of a catalyst. This system monitors the NOx and CO concentrations at the same time by using a NOx sensor and a CO sensor. A resistance value of the NOx sensor changes in accordance with NOx concentrations by using semiconductor, e.g., SnO.sub.2, and a resistance value of the CO sensor changes in accordance with CO concentrations by using SnO.sub.2 in the same manner. Both the NOx sensor and the CO sensor are provided downstream of the catalyst. The air fuel ratio is corrected to a rich direction if the NOx concentration is greater than a predetermined value because NOx concentration increases when the oxygen sensor deteriorated to a lean side. The air fuel ratio is corrected to a lean direction if the CO concentration is greater than a predetermined value because CO or HC concentration increases when the oxygen sensor deteriorated to a rich side. Accordingly, the NOx, CO or HC concentration can be minimized even if the oxygen sensor deteriorates.
In this system, however, the air fuel ratio control is not subjected to correction if the NOx, CO and HC concentration is equal to or less than a predetermined value in order to control an increase in harmful gas component due to the deterioration of an oxygen sensor, the control point for air fuel ratio varied to a great extent in the window of a three-way catalyst though the harmful gas component is below a regulating value. Additionally, there was a drawback that a sensor for measuring NOx, component on the lean side and another sensor for measuring CO or HC, component on the rich side are required (in addition to an oxygen sensors, two gas sensor are required) because it is not obvious whether an oxygen sensor deteriorates to the rich side or to the lean side. Thus, this system had critical problems such as the need for two gas sensors and correction control function for two sensors, dispersion in harmful components, and excessive cost.
In addition, this system has another drawback that the deterioration of a catalyst cannot be detected.